LPCVD is a technique for forming thin film layers on substrates. In particular, LPCVD is used to form metal films (e.g., for wiring), semiconductor films (e.g., for doping to form active areas) and insulating films (e.g., for insulating the metal wires) on silicon wafers in fabricating semiconductor devices such as integrated circuits.
Briefly, a typical prior art LPCVD apparatus includes a heated chamber within which is held one or more wafers on which a thin film is to be deposited. A gas inflow system at one end of the chamber inflows a reactive gas into the chamber. The chamber is kept at low pressure so that the reactive gas flows through the chamber and flows over the surfaces of the one or more wafers. The combination of temperature, pressure and the particular type of reactive gas results in a film being deposited on the surface of each of the one or more wafers. A gas exhaust system removes the reactive gas, as well as other gaseous compounds formed by the reactive process.
In semiconductor manufacturing, it is highly advantageous to deposit thin films on a plurality of wafers in one deposition process because it increases throughput and allows for better process control. However, a major disadvantage with this approach using prior art LPCVD apparatus is that the uniformity of the films deposited on each wafer can vary over each wafer and amongst the wafers due to uneven gas flow velocity between the wafers. This is because the deposition rate of the film is related to the uniformity of the gas flow velocity between the wafers. In particular, it has been observed that the wafers closest the gas input end typically have non-uniform thin films. This is problematic, since in the art of semiconductor manufacturing, it is known that uniform films are an important aspect of successfully fabricating semiconductor devices.
Most prior art apparatus are capable of altering the gas flow within the chamber only by altering the amount of exhaust using a vacuum pump, or by altering the gas inflow at the gas inflow end of the chamber. For example, U.S. Pat. No. 4,699,805 to Seelbach et al. discloses an LPCVD process and apparatus for enhancing the gas flow within the chamber by providing a U-shaped gas injection tube. The tube is coupled to a vacuum pump which controls the pressure within the chamber. Another example of external chamber pressure control is described in U.S. Pat. No. 5,653,810 to Kataoka et al., which discloses an apparatus and process for forming a metal film in a CVD apparatus, wherein the pressure inside the liner tube is controlled by a control valve in an external exhaust unit.
Other prior art CVD apparatus are capable of altering the flow of gas within the chamber, but are limited to forming a film on a single wafer. For example, U.S. Pat. No. 5,362,526 to Wang et al. discloses a high-pressure single wafer CVD apparatus having a uniform radial-pumping gas apparatus which enables uniform reactant gas flow across the wafer. The gas apparatus includes a gas distributor plate in close proximity to the wafer. While the distributor plate is directed to the general idea of directing the gas flow in relation to the wafer, the direction of the flow radial. In addition, the distributor plate is designed for use in a high-pressure, single wafer CVD apparatus, and does not appear to be applicable to a low-pressure, multi-wafer CVD apparatus. Similarly, U.S. Pat. No. 4,798,165 to deBoer discloses an improved technique for providing deposition materials to a growth surface by passing a reactive gas through a plate having a plurality of apertures, the plate being arranged in close proximity to the surface on which deposition is to be carried out. However, the aperture plate is limited to affecting the air flow around in the vicinity of the single surface. Moreover, the plate is required to be substantially the size of the single surface.